This invention generally relates to liquid-metal ion sources for use in field emission-type ion beam generating devices, and more particularly, to a high-wetting-capability, liquid-metal ion source for use in field-emission-type, ion-beam generating devices employing carbon (graphite) substrates, and to a substrate wetting method employing the source. Still more particularly, it relates to such a source which takes the form of a binary alloy including a predominance of boron, which alloy employs a sufficient enrichment of boron to promote significant substrate wetting. Still more particularly, it relates to a method for wetting a graphite substrate with a boron alloy for use in generating an ion beam containing boron.
In the recent past, there has been an extensive and increasing interest in the employment of so-called focused ion beams. Such beams are used for various purposes, such as for micromachining of certain materials, and for implantation of ions in the making of semiconductor devices. In the fabrication of semiconductor devices, circuit elements may be formed by photolithographic techniques or by ion implantation. Field emission sources have been developed for ion implantation wherein a liquid metal adheres to a needle point. The extraction voltage applied between the needle and an extraction electrode shapes the liquid at the needle tip into a cusp, or Taylor cone, with an effective emission point with a size less than 50 .ANG.. The liquid-metal ion source must wet and substantially cover the needle surface in order to provide a stable ion source with an acceptable life. One of many elements which offers an important source of ions for such purposes is boron. However, implementing a successful focused beam of boron ions has presented a number of problems.
Boron is a desirable dopant for use in ion implantation. However, elemental boron is not suitable because of its high melting point. Boron alloys of nickel (Ni), palladium (Pd), and platinum (Pt) have been formed but these alloys tend to corrode needle substrate materials which the alloys readily wet and do not readily wet materials, such as graphite, which the alloys do not corrode.
Boron-based ion sources are extremely corrosive to conventional, metallic, point-source support substrates which are employed in ion-beam generating devices. As a consequence, they yield an operational situation in which practical substrate lifetime expectancy is extremely short--like just a few hours. Carbon, typically in the form of graphite, offers a "material" solution to the corrosion problem when it is employed as such a support substrate, but frustration has existed over the poor ability of known boron-based ion sources to wet such a substrate well enough to function properly as a practical source.
The prior art has made several attempts to solve the wetting problem. In one attempt boron is used to precoat the graphite surface. The boron may be applied to the surface as a boron powder. The boron coating is conventionally heated to a high temperature, about 2500 K, to react the boron with the surface.
Boron in the form of red boron has also been added directly to a eutectic NiB alloy as a flux. The mixture is heated to melting on a graphite surface. This mixture produces some wetting of the graphite surface, but the results are not uniform and predictable, possibly because of the uneven distribution of boron flux.